Comment by wlonkly

There's three players: registry, registrar, and DNS host. The parts that affect your availability are the registry's nameservers and your DNS host. The registrar is the control plane for the registry -- if your registrar is unavailable and your registry and DNS host are fine, you're still up.

If your DNS host is unavailable _and_ they're your registrar, then you can't make changes at the registry to use a different DNS host.

But if they're unavailable and they're not your registrar, then you can still make changes at the registry to use a different DNS host (although you'll still have to deal with the two-day TTL on the NS records at the root).

So the idea of using a different registrar and DNS host amounts to separating the control and data plane for your domain.